Characterization of self-assembled monolayers of porphyrins bearing multiple thiol-derivatized rigid-rod tethers

A series of multithiol-functionalized zinc porphyrins has been prepared and characterized as self-assembled monolayers (SAMS) on Au. The molecules, designated ZnPSn (n = 1-4), contain from one to four [(S-acetylthio)methyl]phenylethynylphenyl groups appended to the meso-position of the porphyrin; the other meso-substituents are phenyl groups. For the dithiol-functionalized molecules, both the cis- and the trans-appended structures were examined. The ZnPSn SAMS were investigated using X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and various electrochemical methods. The studies reveal the following characteristics of the ZnPSn SAMS. (1) The ZnPSn molecules bind to the Au surface via a single thiol regardless of the number of thiol appendages that are available per molecular unit. (2) The porphyrins in the ZnPS3 and ZnPS4 SAMS bind to the surface in a more upright orientation than the porphyrins in the ZnPS1, cis-ZnPS2, and trans-ZnPS2 SAMS. The porphyrins in the ZnPS3 and ZnPS4 SAMS are also more densely packed than those in the cis-ZnPS2 and trans-ZnPS2 SAMS. The packing density of the ZnPS3 and ZnPS4 SAMS is similar to that of the ZnPS1 SAMS, despite the larger size of the molecules in the former SAMS. (3) The thermodynamics and kinetics of electron transfer are generally similar for all of the ZnPSn SAMS. The general similarities in the electron-transfer characteristics for all of the SAMS are attributed to the similar binding motif.